1. Field of the Invention
This invention is in the field of energy and more particularly relates to aqueous heat-storage compositions suitable for absorbing and storing energy as they are heated above a predetermined temperature and subsequently releasing heat energy as they are cooled below the predetermined temperature.
2. Description of the Prior Art
Phase-change materials that absorb and release heat latently when they are cycled above and below a given temperature have been attractive candidates for thermal storage because of the great quantities of heat that can be stored in a relatively small volume of material. Although there is a large number of materials which are suitable, based upon their latent heats of fusion and melting points, the overwhelming majority of such materials fail to meet other stringent physical and chemical requirements necessary for practical systems.
Organic materials, for example, have a tendency to deteriorate over relatively short periods of time and are also susceptible to growth of microorganisms. Additionally, many inorganic and organic materials are too expensive to be practical, have unacceptable coefficients of thermal expansion, toxicities or other physical or chemical properties.
Of all the possible materials, inorganic salt hydrates have proven to have the best balance of properties to date. Unfortunately, such inorganic salt hydrates often lose their heat-storage efficiency as the materials are repeatedly thermally cycled. Often, such loss of heat-storage efficiency is associated with incongruent melting of components which produces phase separation in these materials after an insufficient number of thermal cyclings.
Several attempts to overcome phase separation after repeated thermal cycling due to incongruous melting of inorganic salt hydrates are described in the patent literature.
Laing et al., in U.S. Pat. No. 3,720,198, for example, describe shape-retaining bodies which may be in the form of a sponge, foam, or zeolite-type structure, which bodies are insoluble or substantially insoluble in melted storage material at the operating temperature and which have a plurality of capillary passages. Such shape-retaining bodies retain molten heat-storage materials by capillary forces. Zeolite-type structures which retain their shape are formed by intimately mixing a heat-storage material, body-forming material and seed crystal; heating the mixture to a temperature where the body-forming material is 30% soluble; and cooling to form a solid shape-retaining body. Inorganic salt hydrates, including the decahydrate of sodium sulfate or the dodecahydrate of disodium hydrogen phosphate, are disclosed as suitable heat-storage materials. Particularly suitable substances for producing the body are stated to be those which have a fibrous crystal structure. These include fibrous or flocculent silicates, such as aerosil (fumed silicon dioxide), tripotassium or dipotassium silicate, calcium aluminates, the ferrites of the light metals, flocculent soot, magnesium oxide, silicon oxide and other flocculent crystals of other oxides or salts.
Telkes, in U.S. Pat. No. 3,986,969, discloses aqueous dispersions of heat-of-fusion materials, including sodium sulfate decahydrate and eutectics thereof, with a nucleating agent, such as borax, and a homogenizing agent comprising clay-type substances made up of particles that are lath-like in appearance. Telkes' preferred homogenizing or thickening agents are attapulgus-type clays, particularly the attapulgite-type clay sold under the trademark "Min-U-Gel 200" by the Floridin Company, Berkley Springs, West Virginia. These attapulgite-type clays are stated by Telkes to provide increased stability compared to previously used thickening agents, including wood shavings, wood pulp, sawdust, various types of cellulosic mixtures, or METHOCELL thickeners, starch, organic alginates, silica gel, diatomaceous earth, and other finely divided silica products.
Despite the improved stability achieved by Telkes' heat-storage compositions, these compositions still do not have sufficient retention of heat-storage efficiencies to make them practical, and these Telkes' compositions still do undergo phase separation after an insufficient number of thermal cyclings through their phase-change temperature.